1. Field of the Invention
The invention generally relates to a light emitting diode (LED) driving technique, and more particularly, to an LED driving apparatus capable of controlling the flowing current and the operation voltage of LEDs.
2. Description of Related Art
FIG. 1 is a diagram of a conventional LED driving apparatus 10. Referring to FIG. 1, the LED driving apparatus 10 is suitable to drive an LED string 101 comprised by a plurality of LEDs L connected in series. The LED driving apparatus 10 includes a power conversion stage 103, a power transistor Q, a resistor Rcs, an operational amplifier OP, an error amplifier EA, a switch SW, two current sources I1 and I2 and two bipolar junction transistors (BJTs) B1 and B2.
In general, the predetermined voltage VSET received by the positive input terminal (+) of the operational amplifier OP would determine the current flowing through the LED string 101. In this way, the operational amplifier OP can compare a detection voltage Vcs with the predetermined voltage VSET so as to switch the power transistor Q and thereby to keep the current flowing through the LED string 101 as a constant-current. On the other hand, in order to avoid the LED driving apparatus 10 during an operation under constant-current from having excessive power loss (i.e., a product of the current flowing though the LED string 101 and the voltage at the node N1), the control voltage VCTR output from the error amplifier EA is used to control the level of the DC voltage VBUS provided by the power conversion stage 103 for the LED string 101, so that the voltage at the node N1 (i.e., the drain voltage of the power transistor Q) is reduced.
To be specific, the switch SW would be turned on during constant-current operations so that the error amplifier EA would compare the voltage at the node N1 with the reference voltage Vref at the node N3 and then perform error amplification, so as to output the control voltage VCTR to control the level of the DC voltage VBUS provided by the power conversion stage 103. It can be seen from the depiction above that in the conventional LED driving apparatus 10, the level of the DC voltage VBUS provided by the power conversion stage 103 is controlled by a feedback from the drain of the power transistor Q.
However, the architecture of the conventional LED driving apparatus 10 has following problems:
1. A basic voltage Vledmin for determining the reference voltage Vref must vary with the change of the predetermined voltage VSET (i.e., to change the level of the reference voltage Vref);
2. Since the Rds-on of the power transistor Q would be increased with the rise of the temperature during the power transistor Q is turned on (the Rds-on behaves with positive temperature coefficient) so that the basic voltage Vledmin for determining the reference voltage Vref must vary with the change of the temperature (i.e., to change the level of the reference voltage Vref), and thus making the control mechanism of the LED driving apparatus 10 relatively complicated; and
3. During the process of non-dimming (at the time, the current flowing through the LEDs L is zero), since the voltage at the node N1 is a relatively high level (normally, tens of volts), the switch SW must be turned off to avoid the internal components of the LED driving apparatus 10 from damage. Meanwhile, the switch SW should be a high-voltage element.